The present invention relates to wires for connecting integrated circuit (IC) die to leads and wirebonding, and more particularly, to a method of wirebonding insulated wires.
An IC die is a small device formed on a semiconductor wafer, such as a silicon wafer. Such a die is typically cut from the wafer and attached to a substrate or base carrier for interconnect redistribution. Bond pads on the die are electrically connected to leads on the substrate with wires via wire bonding. Wires may also be used to cross connect bond pads of the die or to cross connect leads of the substrate. The die, wires and substrate are then encapsulated to form a packaged device.
There is a continuing demand for more dense integrated circuits, yet without a corresponding increase in the size or footprint of the packaged device. There is also a desire for more inputs and outputs to ICs, resulting in high densities of connections between the IC die and the substrate, and the need for fine pitch and ultra-fine pitch wire bonding. The diameter of the bond wire also has decreased. For example, 63 um pitch applications use 25 um diameter wire, while 52 um and 44 um pitch applications use 20.3 um diameter wire. Development is being done for a 37 um pitch application using 17 um diameter wire.
Decreases in pitch and wire diameter cause difficulties in handling and wire bonding. For example, wires may unintentionally short to other conductive structures of the packaged device, such as other wires, pads, leads, or the die. This shorting may occur during IC die encapsulation as, for example, from “sweeping,” where the injection or transfer of the liquid molding encapsulant moves the wires against another conductive structure. Parts that use smaller diameter wires tend to have higher wire sweep rejects. Using insulated or coated wires may decrease such wire sweep and shortings. However, it is difficult to obtain good bonding quality when using coated wires, especially for the second bond. That is, a wirebonder makes a first bond at the die bond pad and a second bond at the carrier lead. Non-sticking on the lead is a common problem with insulated wire. One attempt at overcoming the weak second bond was to remove the insulator using an electric flame off (EFO). However, performing EFO at the second bond requires a special wirebonding machine and takes additional time.
The present invention provides a method of wire bonding insulated or coated wire using a standard wirebonder that improves the bonding quality of the second bond.